The dynamic state of thymus derived (T) and bone marrow derived (B) lymphocytes has been delineated in mice lightly infected with S. mansoni. Additionally, lung granulomatous response around Schistosome eggs in T lymphocyte deprived and fully reconstituted animals has been investigated. (BA)J mice infected with 10 cercariae of S. mansoni were tested between 1-32 weeks of the infection for T and B cell distribution using anti-theta serum for T and anti-Ia serum for B cell characterization. Age-matched, normal mice served as controls. No significant change was seen in the population during the first 4 week of the infection. At 8 weeks a decrease in the percentage of T lymphocytes was visible in the spleen and lymph nodes which was accompanied by elevated levels of B cells. Declining T and increasing B cell ratios was observed also in the blood. While at 8 weeks granulomas contained only T cells, subsequently B cells gradually increased in numbers and comprised about half the lymphoid population of the lesions. Between 20-32 weeks and T and B lymphocyte balance slowly returned to normal. B mice deprived of their T lymphocytes were practically incapable of forming primary lung granulomas around eggs, while fully reconstituted animals reacted as control animals.